1. Field of the Invention
The present invention relates to an organic electroluminescent device (EL device) which can emit light by converting electric energy into optical energy.
2. Description of the Related Art
Since an organic electroluminescent device can emit high-brightness light with a low voltage, the organic electroluminescent device has attracted attentions as a promising display device. External quantum efficiency is an important characteristic of the organic electroluminescent device. The external quantum efficiency is calculated as “external quantum efficiency Φ=the number of photons emitted from a device/the number of electrons injected into the device,” and it can be said that it is more advantageous in view of power consumption as the external quantum efficiency becomes greater.
The external quantum efficiency of the organic electroluminescent device is defined by “external quantum efficiency Φ=internal quantum efficiency×light extraction efficiency.” In an organic electroluminescent device using fluorescent light emission from an organic compound, since the limit value of the internal quantum efficiency is about 25% and the light extraction efficiency is about 20%, the limit value of the external quantum efficiency is about 5%.
A technology, which can enhance the internal quantum efficiency of an organic electroluminescent by the use of a triplet light emitting material (phosphorescent material) including a platinum complex and enhance the external quantum efficiency in comparison with a conventional device (singlet light emitting device) using fluorescent light emission, is disclosed in WO 2004/108857